Nitride semiconductor device and method for fabricating the same

ABSTRACT

A nitride semiconductor device includes: first through third nitride semiconductor layers formed in sequence over a substrate. The second nitride semiconductor layer has a band gap energy larger than that of the first nitride semiconductor layer. The third nitride semiconductor layer has an opening. A p-type fourth nitride semiconductor layer is formed so that the opening is filled therewith. A gate electrode is formed on the fourth nitride semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATION

The disclosure of Japanese Patent Application No. 2007-318058 filed inJapan on Dec. 10, 2007 including specification, drawings and claims isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to nitride semiconductor devices andmethod for fabricating the nitride semiconductor devices, andparticularly relates to a power transistor for use in, for example, apower supply circuit and a method for fabricating the power transistor.

In recent years, field effect transistors (FETs) made of gallium nitride(GaN)-based semiconductors have been intensively studied ashigh-frequency high-power devices. III-V nitride semiconductors such asGaN form various types of mixed crystal together with aluminum nitride(AlN) and indium nitride (InN). Thus, GaN-based semiconductors formheterojunctions in the same manner as conventionally-used arsenic-basedsemiconductors such as gallium arsenide (GaAs).

In particular, a heterojunction of a nitride semiconductor has acharacteristic in which spontaneous polarization and/or piezoelectricpolarization produces a high concentration of carriers at the interfacethereof even when the nitride semiconductor is not doped. As a result,FETs using nitride semiconductors are likely to exhibit depletion-mode(normally-on) characteristics, and thus it is difficult to obtainenhancement-mode (normally-off) characteristics. However, most of thedevices currently used in the power electronics market are normally-offdevices, and normally-off operation is also strongly demanded for FETsusing GaN-based semiconductors.

Normally-off FETs are implemented with, for example, a structure inwhich a gate region is recessed so that the threshold voltage ispositively shifted (see, for example, T. Kawasaki et al., “Solid StateDevices and Materials 2005 tech. digest”, 2005, p. 206). A method withwhich a FET is fabricated on the (10-12) plane of a sapphire substrateso that no polarization electric field is generated in the crystalgrowth direction of nitride semiconductor (see, for example, M. Kurodaet al., “Solid State Devices and Materials 2005 tech. digest”, 2005, p.470) is also known. In addition, as a promising structure for obtaininga normally-off FET, a junction field effect transistor (JFET) in which ap-type GaN layer is formed as gate is proposed (see, for example,Japanese Laid-Open Patent Publication No. 2005-244072). In a JFETstructure, piezoelectric polarization produced at the heterointerfacebetween a channel layer of undoped GaN and a bather layer of AlGaN iscanceled by piezoelectric polarization produced at the heterointerfacebetween the barrier layer of AlGaN and a p-type GaN layer. Thisstructure reduces the concentration of two-dimensional electron gasimmediately under a gate region where the p-type GaN layer is formed,thus obtaining normally-off characteristics. The use of a pn junctionhaving a larger built-in potential than that of a Schottky junction forgate advantageously reduces gate leakage current even with anapplication of a positive gate voltage.

SUMMARY OF THE INVENTION

The present inventors actually fabricated the conventional JFETdescribed above to find the problem of occurrence of so-called currentcollapse. Specifically, when the gate is switched from OFF to ONimmediately after an application of a high drain voltage, drain currentdecreases and the ON resistance increases, as compared to the case wherelower drain voltage is applied. The increase in ON resistance due to thecurrent collapse is a serious problem in power transistors to which highdrain voltages are applied.

It is therefore an object of the present invention to suppressoccurrence of current collapse to implement a normally-off nitridesemiconductor device applicable to a power transistor.

To achieve the object, a nitride semiconductor device according to thepresent invention has a structure in which a nitride semiconductor layerhaving a gate recess is formed on a nitride semiconductor layer forminga heterojunction interface and a p-type semiconductor layer is formed inthe gate recess.

Specifically, a nitride semiconductor device according to the presentinvention includes: a substrate; a first nitride semiconductor layerformed on the substrate; a second nitride semiconductor layer formed onthe first nitride semiconductor layer and having a band gap energylarger than that of the first nitride semiconductor layer; a thirdnitride semiconductor layer formed on the second nitride semiconductorlayer and having an opening; a p-type fourth nitride semiconductor layerfilling the opening; and a gate electrode formed on the fourth nitridesemiconductor layer.

In the inventive nitride semiconductor device, a channel layer formedbetween the first nitride semiconductor layer and the secondsemiconductor layer is kept away from the surface. Accordingly, unlike aconventional nitride semiconductor device, a noticeable advantage inwhich the influence of the surface state on the channel layer is reducedand occurrence of current collapse resulting from the surface state issuppressed is obtained. In addition, since the second and third nitridesemiconductor layers are both made of nitride semiconductor, theselayers are allowed to be continuously grown. Accordingly, no interfacestate is formed in the interface between the second nitridesemiconductor layer and the third nitride semiconductor layer so thatthe influence of the interface between the second and third nitridesemiconductor layers does not need to be taken into consideration. Withthis structure, occurrence of current collapse is more effectivelysuppressed. Moreover, since the gate electrode is formed on the p-typefourth nitride semiconductor layer, the concentration of two-dimensionalelectron gas immediately under the gate electrode is selectively lowerthan that in the other region. As a result, normally-off characteristicsare obtained. It is also possible to increase the range of the thresholdvoltage, thus implementing a threshold voltage of about +1V.

A method for fabricating a nitride semiconductor device includes thesteps of: (a) epitaxially growing a first nitride semiconductor layer, asecond nitride semiconductor layer having a band gap energy larger thanthat of the first nitride semiconductor layer and a third nitridesemiconductor layer in sequence over a substrate; (b) selectivelyremoving the third nitride semiconductor layer to form an opening; (c)epitaxially growing a p-type fourth nitride semiconductor layer to fillthe opening therewith; and (d) forming a gate electrode on the fourthnitride semiconductor layer.

The inventive method for fabricating a nitride semiconductor deviceincludes the step of epitaxially growing first through third nitridesemiconductor layers in sequence. Thus, no interface state is formed inthe interface between the second nitride semiconductor layer and thethird nitride semiconductor layer. In addition, channel formed betweenthe first nitride semiconductor layer and the second nitridesemiconductor layer is kept away from the surface. Accordingly, theinfluence of the surface state on the channel layer is reduced, thusmuch more effectively suppressing occurrence of current collapseresulting from the surface state, than in a conventional method. Theinventive method also includes the step of epitaxially growing a p-typefourth nitride semiconductor layer so that an opening is filledtherewith. Thus, a normally-off nitride semiconductor device is easilyfabricated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a nitride semiconductordevice according to a first embodiment of the present invention.

FIGS. 2A through 2C are cross-sectional views showing respective processsteps of a method for fabricating a nitride semiconductor deviceaccording to the first embodiment in the order of fabrication.

FIGS. 3A through 3C are cross-sectional views showing respective processsteps of the method for fabricating a nitride semiconductor device ofthe first embodiment in the order of fabrication.

FIG. 4 is a cross-sectional view illustrating a nitride semiconductordevice according to a second embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a nitride semiconductordevice according to a first modified example of the second embodiment.

FIG. 6 is a cross-sectional view illustrating a nitride semiconductordevice according to a second modified example of the second embodiment.

FIG. 7 is a cross-sectional view illustrating a nitride semiconductordevice according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

A first embodiment of the present invention will be described withreference to the drawings. FIG. 1 is a cross-sectional view illustratinga nitride semiconductor device according to the first embodiment. Abuffer layer 12 made of AlN and having a thickness of 100 nm, a firstnitride semiconductor layer 13 made of undoped GaN and having athickness of 2 μm, a second nitride semiconductor layer 14 made ofundoped AlGaN and having a thickness of 15 nm, and a third nitridesemiconductor layer 15 made of n-type AlGaN and having a thickness of 30nm are formed on a c-plane sapphire substrate 11. The gate region of thethird nitride semiconductor layer 15 has an opening in which the secondnitride semiconductor layer 14 is exposed. A fourth nitridesemiconductor layer 16 with a thickness of 100 nm is formed above thethird nitride semiconductor layer 15 to fill the opening. A fifthnitride semiconductor layer 17 made of undoped AlGaN and having athickness of 5 nm is formed between the fourth nitride semiconductorlayer 16 and the second and third nitride semiconductor layers 14 and15. The term “undoped” herein means that no impurities are intentionallyintroduced.

A gate electrode 21 of palladium (Pd) is formed on the fourth nitridesemiconductor layer 16. The gate electrode 21 is in ohmic contact withthe fourth nitride semiconductor layer 16. Source/drain electrodes 22and 23 are respectively formed at both sides of the gate electrode 21.In this embodiment, the source/drain electrodes 22 and 23 are formed inrecesses formed by removing the fourth nitride semiconductor layer 16,the fifth nitride semiconductor layer 17, and parts of the third, secondand first nitride semiconductor layers 15, 14 and 13. In this manner,the source/drain electrodes 22 and 23 are in direct contact withtwo-dimensional electron gas produced near the heterojunction interfacebetween the first nitride semiconductor layer 13 and the second nitridesemiconductor layer 14, thereby reducing the contact resistance. Thesource/drain electrodes 22 and 23 are a stack of a titanium (Ti) layerand an aluminum (Al) layer. The source/drain electrodes 22 and 23 arenot necessarily formed in recesses, and only need to be in ohmic contactwith the two-dimensional electron gas serving as channel.

Most part of the fourth nitride semiconductor layer 16 is doped withapproximately 1×10¹⁹ cm⁻³ of magnesium (Mg). Thus, the carrierconcentration is approximately 1×10¹⁸ cm⁻³. However, a regionimmediately under the gate electrode to a depth of approximately 10 nmis doped with approximately 1×10²⁰ cm⁻³ of Mg. The carrier concentrationof the third nitride semiconductor layer 15 is approximately 1×10¹⁸cm⁻³.

In the nitride semiconductor device of this embodiment, thetwo-dimensional electron gas concentration immediately under the gateelectrode is lower than that in the other region so that normally-offoperation is implemented. In addition, because of the presence of thethird nitride semiconductor layer 15, channel is kept away from thesurface of the semiconductor layers between the gate electrode 21 andthe source electrode 22 and between the gate electrode 21 and the drainelectrode 23. Accordingly, channel is not susceptible to the influenceof the surface state occurring in the surface of the semiconductorlayers. As a result, occurrence of current collapse is suppressed.

The current collapse is considered to be due to electrons trapped in thesurface state. In the absence of the third nitride semiconductor layer15, with application of a high drain bias of about several tens ofvoltages in the OFF state, the two-dimensional electron gas between thegate and the drain would be depleted by electrons trapped in the surfacestate of the second nitride semiconductor layer 14. Since the timenecessary for electron emission from the surface state is longer thanthat for electron capture, a depletion layer is also formed between thegate and the drain immediately after the gate is turned ON state.Accordingly, it is considered that channel does not fully open so thatthe channel resistance increases.

On the other hand, in the nitride semiconductor device of thisembodiment including the third nitride semiconductor layer 15, thedistance between the channel and the surface state is large. Thus, evenwith an application of a high drain bias in the ON state, no depletionregion is formed in the two-dimensional electron gas between the gateand the drain. Accordingly, the channel fully opens immediately afterthe gate is turned ON state, resulting in that the channel resistancedoes not increase.

To suppress occurrence of current collapse more effectively, thethickness of the third nitride semiconductor layer 15 is preferably 15nm or more, and the carrier concentration of the third nitridesemiconductor layer 15 is preferably 1×10¹⁷ cm⁻³ or more. The thirdnitride semiconductor layer is preferably of the n-type. In this case,the concentration of the two-dimensional electron gas is increased. Inaddition, the current collapse is more effectively suppressed. The thirdnitride semiconductor layer 15 may be undoped.

With such a structure, the two layers, i.e., the second nitridesemiconductor layer 14 of undoped AlGaN and the third nitridesemiconductor layer 15 of n-type AlGaN, are present above the channel,between the gate electrode 21 and the source electrode 22 or between thegate electrode 21 and the drain electrode 23. Accordingly, thecapability of supplying electrons to the channel is enhanced so that thetwo-dimensional electron gas concentration in the channel is selectivelyincreased, thus reducing the channel resistance. As a result, in thenormally-off FET, it is possible to suppress occurrence of currentcollapse, while reducing the ON resistance.

In addition, since the second nitride semiconductor layer 14 and thethird nitride semiconductor layer 15 are both nitride semiconductor,these layers are allowed to be continuously formed, thereby forming nosurface state in the interface between these layers. Thus, electrons aretrapped only in the surface state in the third nitride semiconductorlayer 15, resulting in effectively suppressing occurrence of currentcollapse.

The fourth nitride semiconductor layer 16 forming the gate region isformed at a position closer to the source electrode 22. This is becausethe distance between the gate electrode 21 and the drain electrode 23 isincreased to reduce an electric field occurring with an application of ahigh drain voltage so that the breakdown voltage of the transistor isincreased.

The nitride semiconductor device of this embodiment includes the fifthnitride semiconductor layer 17. If the fourth nitride semiconductorlayer 16 was re-grown directly on the second nitride semiconductor layer14, the difference in lattice constant between AlGaN and GaN might causea lattice mismatch. Accordingly, gate leakage current would increasewith deterioration of crystallinity of the fourth nitride semiconductorlayer 16. On the other hand, in this embodiment, the fifth nitridesemiconductor layer 17 made of AlGaN, which is also used for the secondnitride semiconductor layer 14, is re-grown so that a lattice mismatchin the re-growth interface is reduced. Accordingly, the fourth nitridesemiconductor layer 16 of GaN is formed successively after the formationof the fifth nitride semiconductor layer 17 so that a lattice mismatchoccurring at the re-growth is reduced, thus forming the fourth nitridesemiconductor layer 16 with excellent crystallinity. The fifth nitridesemiconductor layer 17 is preferably a layer having a composition whichis not likely to cause a lattice mismatch with the second nitridesemiconductor layer 14. Specifically, the fifth nitride semiconductorlayer 17 is preferably made of a material having a lattice constantdifference between the fifth nitride semiconductor layer 17 and thesecond nitride semiconductor layer 14 smaller than that between thefourth nitride semiconductor layer 16 and the second nitridesemiconductor layer 14. The fifth nitride semiconductor layer 17 is notnecessarily undoped and may be of the p-type.

Hereinafter, a method for fabricating a nitride semiconductor deviceaccording to the first embodiment will be described. First, as shown inFIG. 2A, a buffer layer 12 made of AlN and having a thickness of 100 nm,a first nitride semiconductor layer 13 made of undoped GaN and having athickness of 2 μm, a second nitride semiconductor layer 14 made ofundoped AlGaN and having a thickness of 15 nm, and a third nitridesemiconductor layer 15 made of n-type AlGaN and having a thickness of 30nm are epitaxially grown in sequence on a c-plane sapphire substrate 11by metal organic chemical vapor deposition (MOCVD).

Then, as shown in FIG. 2B, part of the third nitride semiconductor layer15 in a gate region is selectively removed, thereby forming an opening15 a which is to serve as a gate recess. The third nitride semiconductorlayer 15 is preferably etched by wet etching with ultraviolet radiationusing approximately 10 mol/l of a potassium hydroxide solution as anetchant. In this etching, the second nitride semiconductor layer 14 madeof undoped AlGaN functions as an etching stopper so that the opening 15a is formed with high reproducibility. In addition, the second nitridesemiconductor layer 14 does not suffer from any etching damage. Theopening 15 a may be formed by dry etching using a gas such as a chlorinegas.

Subsequently, as shown in FIG. 2C, a fifth nitride semiconductor layer17 made of undoped AlGaN and having a thickness of 5 nm and a fourthnitride semiconductor layer 16 made of p-type GaN and having a thicknessof 100 nm are epitaxially grown in sequence by MOCVD. In this process,the fifth nitride semiconductor layer 17 and the fourth nitridesemiconductor layer 16 may be formed only in the gate region using amask of an insulating film such as a silicon oxide film. In this case, asubsequent process of etching the fifth nitride semiconductor layer 17and the fourth nitride semiconductor layer 16 may be omitted.

Thereafter, as shown in FIG. 3A, parts of the fourth nitridesemiconductor layer 16 and the fifth nitride semiconductor layer 17formed in a region except for the gate region are selectively removedby, for example, dry etching with an inductive-coupled plasma (ICP)using a gas such as a chlorine gas.

Then, as shown in FIG. 3B, the third nitride semiconductor layer 15 andparts of the second nitride semiconductor layer 14 and the first nitridesemiconductor layer 13 in a region where source/drain electrodes are tobe formed are selectively removed by, for example, ICP etching using agas such as a chlorine gas, thereby forming recesses 15 b.

Subsequently, as shown in FIG. 3C, a Ti layer and an Al layer are formedin the recesses 15 b, and then a heat process is performed at 650° C. ina nitrogen atmosphere, thereby forming a source electrode 22 and a drainelectrode 23. At last, a gate electrode 21 made of Pd is formed on thefourth nitride semiconductor layer 16.

Embodiment 2

Now, a second embodiment of the present invention will be described withreference to the drawing. FIG. 4 is a cross-sectional view showing astructure of a nitride semiconductor device according to the secondembodiment. In FIG. 4, components also shown in FIG. 1 are denoted bythe same reference numerals, and thus description thereof will beomitted.

The nitride semiconductor device of the second embodiment includes asixth nitride semiconductor layer 18 formed between a second nitridesemiconductor layer 14 and a third nitride semiconductor layer 15. Thesixth nitride semiconductor layer 18 is made of p-type GaN and has athickness of 5 nm. The sixth nitride semiconductor layer 18 is dopedwith approximately 1×10¹⁹ cm⁻³ of Mg and has a carrier concentration ofapproximately 1×10¹⁸ cm⁻³.

The sixth nitride semiconductor layer 18 functions as an etching stopperduring wet etching for forming an opening in the third nitridesemiconductor layer 15. In the first embodiment, the undoped secondnitride semiconductor layer 14 functions as an etching stopper duringthe wet etching. In this embodiment, the p-type sixth nitridesemiconductor layer 18 stops wet etching without fail.

In addition, in this embodiment, the sixth nitride semiconductor layer18 and a fourth nitride semiconductor layer 16 are both made of GaN.This eliminates a lattice mismatch occurring when the fourth nitridesemiconductor layer 16 is re-grown on the sixth nitride semiconductorlayer 18. Accordingly, the crystallinity of the fourth nitridesemiconductor layer 16 is enhanced. The sixth nitride semiconductorlayer 18 and the fourth nitride semiconductor layer 16 preferably havethe same composition as in this embodiment. In this case, thecrystallinity of the fourth nitride semiconductor layer 16 is moreeffectively enhanced. However, these layers may have other compositionsas long as a lattice mismatch at the growth interface is reduced.Specifically, the difference in lattice constant between the sixthnitride semiconductor layer 18 and the fourth nitride semiconductorlayer 16 is preferably smaller than that between the second nitridesemiconductor layer 14 and the fourth nitride semiconductor layer 16.

Modified Example 1 of Embodiment 2

Now, a first modified example of the second embodiment will be describedwith reference to the drawing. FIG. 5 is a cross-sectional view showinga structure of a nitride semiconductor device according to the firstmodified example of the second embodiment. In FIG. 5, components alsoshown in FIG. 1 are denoted by the same reference numerals, and thusdescription thereof will be omitted.

The nitride semiconductor device of this modified example includes asixth nitride semiconductor layer 28 formed between the second nitridesemiconductor layer 14 and the third nitride semiconductor layer 15 andmade of p-type AlGaN. In this structure, the AlGaN layer having a largeband gap forms a pn junction interface. Accordingly, gate leakagecurrent is reduced.

In this modified example, the fifth nitride semiconductor layer 17 madeof p-type AlGaN is formed in order to enhance the crystallinity of thefourth nitride semiconductor layer 16. However, the fifth nitridesemiconductor layer 17 may be omitted. The lattice mismatch between thefifth nitride semiconductor layer 17 and the sixth nitride semiconductorlayer 28 is preferably small. Specifically, the difference in latticeconstant between the sixth nitride semiconductor layer 28 and the fifthnitride semiconductor layer 17 is preferably smaller than that betweenthe sixth nitride semiconductor layer 28 and the fourth nitridesemiconductor layer 16.

To reduce gate leakage current, the sixth nitride semiconductor layer 28preferably has a band gap larger than that of the fourth nitridesemiconductor layer 16. The sixth nitride semiconductor layer 28preferably has the same Al content as the second nitride semiconductorlayer 14. In this case, the sixth nitride semiconductor layer 28 iseasily formed. However, the sixth nitride semiconductor layer 28 and thesecond nitride semiconductor layer 14 may have different Al contents.For example, the composition of the second nitride semiconductor layer14 may be Al_(0.15)Ga_(0.85)N and the composition of the sixth nitridesemiconductor layer 28 may be Al_(0.10)Ga_(0.90)N.

Modified Example 2 of Embodiment 2

Now, a second modified example of the second embodiment will bedescribed with reference to the drawing. FIG. 6 is a cross-sectionalview showing a structure of a nitride semiconductor device according tothe second modified example of the second embodiment. In FIG. 6,components also shown in FIG. 1 are denoted by the same referencenumerals, and thus description thereof will be omitted.

The nitride semiconductor device of this modified example includes asixth nitride semiconductor layer 38 formed between the second nitridesemiconductor layer 14 and the third nitride semiconductor layer 15. Thesixth nitride semiconductor layer 38 includes a first p-type layer 38Amade of p-type AlGaN and having a thickness of 5 nm and a second p-typelayer 38B made of GaN and having a thickness of 5 nm.

In the nitride semiconductor device of this modified example, the pnjunction interface is made of the AlGaN layer having a large band gap sothat gate leakage current is reduced. In addition, the fourth nitridesemiconductor layer 16 and the second p-type layer 38B have the samecomposition so that no lattice mismatch occurs at the growth interfaceduring the epitaxial growth of the fourth nitride semiconductor layer16. Accordingly, the crystallinity of the fourth nitride semiconductorlayer 16 is enhanced.

To reduce gate leakage current, the first p-type layer 38A preferablyhas a band gap larger than that of the second p-type layer 38B. Thefirst p-type layer 38A preferably has the same Al content as the secondnitride semiconductor layer 14. In this case, the first p-type layer 38Ais easily formed. However, the first p-type layer 38A and the secondnitride semiconductor layer 14 may have different Al contents. Forexample, the composition of the second nitride semiconductor layer 14may be Al_(0.15)Ga_(0.85)N and the composition of the second p-typelayer 38B may be Al_(0.10)Ga_(0.90)N.

The second p-type layer 38B and the fourth nitride semiconductor layer16 preferably have the same composition. In this case, the crystallinityof the fourth nitride semiconductor layer 16 is more effectivelyenhanced. However, these layers may have different compositions as longas a lattice mismatch at the growth interface is reduced. Specifically,the difference in lattice constant between the second p-type layer 38Band the fourth nitride semiconductor layer 16 is preferably smaller thanthat between the first p-type layer 38A and the fourth nitridesemiconductor layer 16.

Embodiment 3

Now, a third embodiment of the present invention will be described withreference to the drawing. FIG. 7 is a cross-sectional view showing astructure of a nitride semiconductor device according to the thirdembodiment. In FIG. 7, components also shown in FIG. 1 are denoted bythe same reference numerals, and thus description thereof will beomitted.

The nitride semiconductor device of the third embodiment includes asixth nitride semiconductor layer 19 between a second nitridesemiconductor layer 14 and a third nitride semiconductor layer 15. Thesixth nitride semiconductor layer 19 is made of n-type GaN and has athickness of 5 nm.

The sixth nitride semiconductor layer 19 functions as an etching stopperduring wet etching for forming an opening in the third nitridesemiconductor layer 15. In the second embodiment, the p-type sixthnitride semiconductor layer 18 functions as an etching stopper duringthe wet etching. On the other hand, in the third embodiment, though thesixth nitride semiconductor layer 19 is of the n-type, which is the sameconductivity as the third nitride semiconductor layer 15 to be etched,the sixth nitride semiconductor layer 19 has a band gap smaller thanthat of the third nitride semiconductor layer 15, thus enabling stoppingof the wet etching.

In the foregoing embodiments and modified examples, the substrate 11 isa sapphire (0001) substrate. However, the substrate 11 may be made ofany material such as SiC, GaN, or Si. The substrate 11 may have anyorientation as long as excellent crystallinity is obtained.

In the case where the first nitride semiconductor layer 13 is made ofGaN, the composition of the second nitride semiconductor layer 14 isAl_(0.15)Ga_(0.85)N in order to form excellent channel. The An contentthereof may be appropriately changed.

The third nitride semiconductor layer 15 and the second nitridesemiconductor layer 14 may have the same composition or may havedifferent compositions. Instead of AlGaN, either AlN including no Ga orGaN including no Al may be used. In the case of AlN, the layers arepreferably epitaxially grown in an amorphous state at a low temperatureof approximately 500° C. In general, if an AlN layer is formed on athick GaN layer having a thickness of several micrometers, a latticemismatch is large so that cracks easily occur. However, if the AlN layeris grown at a low temperature, no cracks occur to a thickness of about30 nm. In addition, since the AlN layer which has grown at a lowtemperature is easily wet etched with an alkaline solution such as adeveloper, the advantage of easy formation of an opening is obtained.

The fourth nitride semiconductor layer 16 is preferably made of GaN. Inthis case, Mg is easily activated. However, the fourth nitridesemiconductor layer 16 may have another composition.

The thickness of the sixth nitride semiconductor layer may beappropriately changed. However, in the case where the sixth nitridesemiconductor layer is of the p-type, an excessive thickness of thesixth nitride semiconductor layer would cause leakage current to flowthrough the sixth nitride semiconductor layer. Thus, the thickness ofthe sixth nitride semiconductor layer may be in the range where holesare depleted in the region between the gate electrode and the drainelectrode and between the gate electrode and the source electrode.

The gate electrode may be made of other metals such as Ni as long as anexcellent ohmic contact is obtained.

In the foregoing embodiments, GaN and AlGaN are used as nitridesemiconductor materials. However, the present invention is not limitedto these materials. For example, InGaN and AlInGaN may be used. Thethicknesses of the layers may be appropriately changed.

As described above, a nitride semiconductor device and a method forfabricating the nitride semiconductor device according to the presentinvention allow a normally-off nitride semiconductor device in which theON resistance is low and occurrence of current collapse is suppressed tobe implemented and, thus, are useful especially as a power transistorfor use in a power supply circuit in consumer equipment such as atelevision set and a method for fabricating the power transistor, forexample.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1-20. (canceled)
 21. A nitride semiconductor device, comprising: asubstrate; a nitride semiconductor layer made of GaN and formed on thesubstrate; a nitride semiconductor layer made of AlGaN and formed on thenitride semiconductor layer made of GaN; a p-type nitride semiconductorlayer filling an opening formed on the nitride semiconductor layer madeof AlGaN; a gate electrode formed on the p-type nitride semiconductorlayer; and source/drain electrodes respectively formed at sides of thegate electrode, wherein a distance between the opening and the sourceelectrode differs from a distance between the gate electrode and thedrain electrode.
 22. The nitride semiconductor device of claim 21,wherein the nitride semiconductor layer made of AlGaN is of an n-type.23. The nitride semiconductor device of claim 21, wherein the distancebetween the opening and the source electrode is smaller than thedistance between the gate electrode and the drain electrode.
 24. Anitride semiconductor device, comprising: a substrate; a first nitridesemiconductor layer formed on the substrate; a second nitridesemiconductor layer formed on the first nitride semiconductor layer andhaving a band gap energy larger than that of the first nitridesemiconductor layer; a third nitride semiconductor layer formed on thesecond nitride semiconductor layer and having an opening; a p-typefourth nitride semiconductor layer filling the opening; and a gateelectrode formed on the fourth nitride semiconductor layer, wherein adistance between the opening and the source electrode differs from adistance between the opening and the drain electrode.
 25. The nitridesemiconductor device of claim 24, wherein the distance between theopening and the source electrode is smaller than the distance betweenthe opening and the drain electrode.
 26. The nitride semiconductordevice of claim 24, wherein the gate electrode has a width smaller thanthat of the fourth nitride semiconductor layer.
 27. The nitridesemiconductor device of claim 24, wherein the third nitridesemiconductor layer is of an n-type.
 28. The nitride semiconductordevice of claim 24, further including a fifth nitride semiconductorlayer formed between the second nitride semiconductor layer and thefourth nitride semiconductor layer.
 29. The nitride semiconductor deviceof claim 28, wherein a difference in lattice constant between the secondnitride semiconductor layer and the fifth nitride semiconductor layer issmaller than that between the second nitride semiconductor layer and thefourth nitride semiconductor layer.
 30. The nitride semiconductor deviceof claim 24, further including a sixth nitride semiconductor layerformed between the second nitride semiconductor layer and the thirdnitride semiconductor layer.
 31. The nitride semiconductor device ofclaim 30, wherein the sixth nitride semiconductor layer is of a p-type.32. The nitride semiconductor device of claim 31, wherein a differencein lattice constant between the sixth nitride semiconductor layer andthe fourth nitride semiconductor layer is smaller than that between thesecond nitride semiconductor layer and the fourth nitride semiconductorlayer.
 33. The nitride semiconductor device of claim 31, wherein thesixth nitride semiconductor layer has a band gap energy larger than thatof the fourth nitride semiconductor layer.
 34. The nitride semiconductordevice of claim 33, further including a fifth nitride semiconductorlayer formed between the sixth nitride semiconductor layer and thefourth nitride semiconductor layer.
 35. The nitride semiconductor deviceof claim 34, wherein a difference in lattice constant between the sixthnitride semiconductor layer and the fifth nitride semiconductor layer issmaller than that between the sixth nitride semiconductor layer and thefourth nitride semiconductor layer.
 36. The nitride semiconductor deviceof claim 30, wherein the sixth nitride semiconductor layer includes afirst p-type layer and a second p-type layer formed on the first p-typelayer.
 37. The nitride semiconductor device of claim 36, wherein thefirst p-type layer has a band gap energy larger than that of the secondp-type layer, and a difference in lattice constant between the secondp-type layer and the fourth nitride semiconductor layer is smaller thanthat between the first p-type layer and the fourth nitride semiconductorlayer.
 38. The nitride semiconductor device of claim 30, wherein thesixth nitride semiconductor layer is of an n-type.
 39. The nitridesemiconductor device of claim 38, wherein the sixth nitridesemiconductor layer has a band gap energy smaller than that of the thirdnitride semiconductor layer.
 40. The nitride semiconductor device ofclaim 24, wherein the nitride semiconductor device is of a normally-offtype.